The invention relates to an optical device comprising a holder accommodating a system of optical elements, inter alia a lens system, which elements are secured to the inner wall of the holder.
A device of this type is known in many embodiments. Examples of such a device are a telecentric lens system and an optical device in the form of a radiation source unit supplying a collimated radiation beam and having a radiation source and a collimator lens whose object focal plane must coincide with the radiating surface of the radiation source. This radiation source unit is used, for example, in an apparatus for scanning an information plane in an optical record carrier with a radiation spot, to which apparatus the invention also relates.
An apparatus of this type is described in U.S. Pat. No. 4,546,463. An objective system which focuses the parallel beam supplied by the radiation source unit to a scanning spot in the information plane is arranged between the radiation source unit and the record carrier. Scanning an information plane is to be understood to mean both scanning a recorded information plane with a read beam and scanning for the purpose of writing information in this plane with a radiation beam that is intensity-modulated in accordance with the information to be written, as described in U.S. Pat. No. 4,546,463.
In order that an optical record carrier can comprise a sufficiently large quantity of information, the optical details of its information structure must have very small dimensions, of the order of one micrometer. The scanning spot must have a corresponding size. Such a scanning spot is obtained with an objective system having a sufficiently large numerical aperture, at least of the order of 0.40, and whose aperture is adequately filled by the beam in such a manner that a diffraction-limited radiation spot is formed in the information plane. It must be ensured that the scanning spot has a constant size and a constant intensity distribution.
To this end it is not only required that the focal plane of the objective system always coincides with the information plane, which can be realized in various known manners, but also that the beam entering the objective system and coming from the radiation source unit is always parallel and has a constant cross-section. This means that in the radiation source unit the radiating surface of the radiation source must always coincide with the object focal plane of the collimator lens.
The distance between the radiating surface and the collimator lens is determined by the length of the portion of the holder located between these two elements, in which holder the radiation source and the collimator lens are secured. Due to variations of the ambient temperature this length may change so that the radiating surface of the radiation source is moved with respect to the collimator lens. Then the beam supplied by the radiation source unit is no longer optimally parallel. An analogous problem generally presents itself in optical systems whose elements are secured in a holder having a given coefficient of expansion and in which stringent requirements are imposed on the mutual positioning of the optical elements.